Within the prior art, there are two techniques that are utilized to compensate a crystal oscillator that is used in an environment whose temperature varies over a temperature range. One technique is to place the oscillator in an oven where the oven is maintained at a constant temperature; and the second technique is to use a digitally temperature compensated crystal oscillator (DTCXO). Both of these techniques suffer from the disadvantage that as a crystal oscillator is used its frequency shifts due to the mechanical aging of the crystal. This is referred to as crystal aging. A DTCXO uses a microcomputer to compensate a crystal oscillator over a range of temperatures to make the output frequency nominally constant over that temperature range. When the DTCXO is manufactured, the device is stepped through the range of temperatures one degree at a time and given time to thermally stabilize at each temperature. The output frequency is measured and compared against a reference frequency standard. If the frequency is not correct, a compensation value is adjusted by the microcomputer so that the output frequency of the DTCXO is within a given specification to the reference frequency. The digital representation of the compensation value is stored in a nonvolatile digital memory by the microcomputer. When the DTCXO is used in the field, as the temperature varies, the microcomputer uses a temperature sensor which is physically in contact with the crystal oscillator to determine the temperature and then access the compensation values to obtain the compensation value that corresponds to the present temperature. This compensation value is then used to adjust the DTCXO frequency so as to maintain that frequency within the specification. The problem with this technique is as the crystal ages, the stored compensation values are no longer accurate.
The first technique of placing the crystal oscillator in an oven suffers from the following additional problem. In many types of equipment, such as telecommunication switching equipment, the ambient temperature of the telecommunications systems is high so that the oven has to be maintained at an extremely high temperature which increases the aging process, increases the heat dissipated by the oscillator, and deceases the oscillator lifetime.
The prior an has overcome the problem of aging by simply operating crystal oscillators by themselves for many years until the aging effect has diminished. The problems with this technique is the high cost of such crystal oscillators and the fact that, while aging slows, it never stops.